/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.escape;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;

/**
 * An {@link Escaper} that converts literal text into a format safe for inclusion in a particular
 * context (such as an XML document). Typically (but not always), the inverse process of
 * "unescaping" the text is performed automatically by the relevant parser.
 *
 * <p>
 * For example, an XML escaper would convert the literal string {@code
 * "Foo<Bar>"} into {@code "Foo&lt;Bar&gt;"} to prevent {@code "<Bar>"} from being confused with an
 * XML tag. When the resulting XML document is parsed, the parser API will return this text as the
 * original literal string {@code
 * "Foo<Bar>"}.
 *
 * <p>
 * <b>Note:</b> This class is similar to {@link CharEscaper} but with one very important difference.
 * A CharEscaper can only process Java <a href="http://en.wikipedia.org/wiki/UTF-16">UTF16</a>
 * characters in isolation and may not cope when it encounters surrogate pairs. This class
 * facilitates the correct escaping of all Unicode characters.
 *
 * <p>
 * As there are important reasons, including potential security issues, to handle Unicode correctly
 * if you are considering implementing a new escaper you should favor using UnicodeEscaper wherever
 * possible.
 *
 * <p>
 * A {@code UnicodeEscaper} instance is required to be stateless, and safe when used concurrently by
 * multiple threads.
 *
 * <p>
 * Popular escapers are defined as constants in classes like
 * {@link com.google.common.html.HtmlEscapers} and {@link com.google.common.xml.XmlEscapers}. To
 * create your own escapers extend this class and implement the {@link #escape(int)} method.
 *
 * @author David Beaumont
 * @since 15.0
 */
@Beta
@GwtCompatible
public abstract class UnicodeEscaper extends Escaper {
    /** The amount of padding (chars) to use when growing the escape buffer. */
    private static final int DEST_PAD = 32;

    /** Constructor for use by subclasses. */
    protected UnicodeEscaper() {}

    /**
     * Returns the escaped form of the given Unicode code point, or {@code null} if this code point
     * does not need to be escaped. When called as part of an escaping operation, the given code
     * point is guaranteed to be in the range {@code 0 <= cp <= Character#MAX_CODE_POINT}.
     *
     * <p>
     * If an empty array is returned, this effectively strips the input character from the resulting
     * text.
     *
     * <p>
     * If the character does not need to be escaped, this method should return {@code null}, rather
     * than an array containing the character representation of the code point. This enables the
     * escaping algorithm to perform more efficiently.
     *
     * <p>
     * If the implementation of this method cannot correctly handle a particular code point then it
     * should either throw an appropriate runtime exception or return a suitable replacement
     * character. It must never silently discard invalid input as this may constitute a security
     * risk.
     *
     * @param cp the Unicode code point to escape if necessary
     * @return the replacement characters, or {@code null} if no escaping was needed
     */
    protected abstract char[] escape(int cp);

    /**
     * Scans a sub-sequence of characters from a given {@link CharSequence}, returning the index of
     * the next character that requires escaping.
     *
     * <p>
     * <b>Note:</b> When implementing an escaper, it is a good idea to override this method for
     * efficiency. The base class implementation determines successive Unicode code points and
     * invokes {@link #escape(int)} for each of them. If the semantics of your escaper are such that
     * code points in the supplementary range are either all escaped or all unescaped, this method
     * can be implemented more efficiently using {@link CharSequence#charAt(int)}.
     *
     * <p>
     * Note however that if your escaper does not escape characters in the supplementary range, you
     * should either continue to validate the correctness of any surrogate characters encountered or
     * provide a clear warning to users that your escaper does not validate its input.
     *
     * <p>
     * See {@link com.google.common.net.PercentEscaper} for an example.
     *
     * @param csq a sequence of characters
     * @param start the index of the first character to be scanned
     * @param end the index immediately after the last character to be scanned
     * @throws IllegalArgumentException if the scanned sub-sequence of {@code csq} contains invalid
     *         surrogate pairs
     */
    protected int nextEscapeIndex(CharSequence csq, int start, int end) {
        int index = start;
        while (index < end) {
            int cp = codePointAt(csq, index, end);
            if (cp < 0 || escape(cp) != null) {
                break;
            }
            index += Character.isSupplementaryCodePoint(cp) ? 2 : 1;
        }
        return index;
    }

    /**
     * Returns the escaped form of a given literal string.
     *
     * <p>
     * If you are escaping input in arbitrary successive chunks, then it is not generally safe to
     * use this method. If an input string ends with an unmatched high surrogate character, then
     * this method will throw {@link IllegalArgumentException}. You should ensure your input is
     * valid <a href="http://en.wikipedia.org/wiki/UTF-16">UTF-16</a> before calling this method.
     *
     * <p>
     * <b>Note:</b> When implementing an escaper it is a good idea to override this method for
     * efficiency by inlining the implementation of {@link #nextEscapeIndex(CharSequence, int, int)}
     * directly. Doing this for {@link com.google.common.net.PercentEscaper} more than doubled the
     * performance for unescaped strings (as measured by {@link CharEscapersBenchmark}).
     *
     * @param string the literal string to be escaped
     * @return the escaped form of {@code string}
     * @throws NullPointerException if {@code string} is null
     * @throws IllegalArgumentException if invalid surrogate characters are encountered
     */
    @Override
    public String escape(String string) {
        checkNotNull(string);
        int end = string.length();
        int index = nextEscapeIndex(string, 0, end);
        return index == end ? string : escapeSlow(string, index);
    }

    /**
     * Returns the escaped form of a given literal string, starting at the given index. This method
     * is called by the {@link #escape(String)} method when it discovers that escaping is required.
     * It is protected to allow subclasses to override the fastpath escaping function to inline
     * their escaping test. See {@link CharEscaperBuilder} for an example usage.
     *
     * <p>
     * This method is not reentrant and may only be invoked by the top level {@link #escape(String)}
     * method.
     *
     * @param s the literal string to be escaped
     * @param index the index to start escaping from
     * @return the escaped form of {@code string}
     * @throws NullPointerException if {@code string} is null
     * @throws IllegalArgumentException if invalid surrogate characters are encountered
     */
    protected final String escapeSlow(String s, int index) {
        int end = s.length();

        // Get a destination buffer and setup some loop variables.
        char[] dest = Platform.charBufferFromThreadLocal();
        int destIndex = 0;
        int unescapedChunkStart = 0;

        while (index < end) {
            int cp = codePointAt(s, index, end);
            if (cp < 0) {
                throw new IllegalArgumentException("Trailing high surrogate at end of input");
            }
            // It is possible for this to return null because nextEscapeIndex() may
            // (for performance reasons) yield some false positives but it must never
            // give false negatives.
            char[] escaped = escape(cp);
            int nextIndex = index + (Character.isSupplementaryCodePoint(cp) ? 2 : 1);
            if (escaped != null) {
                int charsSkipped = index - unescapedChunkStart;

                // This is the size needed to add the replacement, not the full
                // size needed by the string. We only regrow when we absolutely must.
                int sizeNeeded = destIndex + charsSkipped + escaped.length;
                if (dest.length < sizeNeeded) {
                    int destLength = sizeNeeded + (end - index) + DEST_PAD;
                    dest = growBuffer(dest, destIndex, destLength);
                }
                // If we have skipped any characters, we need to copy them now.
                if (charsSkipped > 0) {
                    s.getChars(unescapedChunkStart, index, dest, destIndex);
                    destIndex += charsSkipped;
                }
                if (escaped.length > 0) {
                    System.arraycopy(escaped, 0, dest, destIndex, escaped.length);
                    destIndex += escaped.length;
                }
                // If we dealt with an escaped character, reset the unescaped range.
                unescapedChunkStart = nextIndex;
            }
            index = nextEscapeIndex(s, nextIndex, end);
        }

        // Process trailing unescaped characters - no need to account for escaped
        // length or padding the allocation.
        int charsSkipped = end - unescapedChunkStart;
        if (charsSkipped > 0) {
            int endIndex = destIndex + charsSkipped;
            if (dest.length < endIndex) {
                dest = growBuffer(dest, destIndex, endIndex);
            }
            s.getChars(unescapedChunkStart, end, dest, destIndex);
            destIndex = endIndex;
        }
        return new String(dest, 0, destIndex);
    }

    /**
     * Returns the Unicode code point of the character at the given index.
     *
     * <p>
     * Unlike {@link Character#codePointAt(CharSequence, int)} or {@link String#codePointAt(int)}
     * this method will never fail silently when encountering an invalid surrogate pair.
     *
     * <p>
     * The behaviour of this method is as follows:
     * <ol>
     * <li>If {@code index >= end}, {@link IndexOutOfBoundsException} is thrown.
     * <li><b>If the character at the specified index is not a surrogate, it is returned.</b>
     * <li>If the first character was a high surrogate value, then an attempt is made to read the
     * next character.
     * <ol>
     * <li><b>If the end of the sequence was reached, the negated value of the trailing high
     * surrogate is returned.</b>
     * <li><b>If the next character was a valid low surrogate, the code point value of the high/low
     * surrogate pair is returned.</b>
     * <li>If the next character was not a low surrogate value, then
     * {@link IllegalArgumentException} is thrown.
     * </ol>
     * <li>If the first character was a low surrogate value, {@link IllegalArgumentException} is
     * thrown.
     * </ol>
     *
     * @param seq the sequence of characters from which to decode the code point
     * @param index the index of the first character to decode
     * @param end the index beyond the last valid character to decode
     * @return the Unicode code point for the given index or the negated value of the trailing high
     *         surrogate character at the end of the sequence
     */
    protected static int codePointAt(CharSequence seq, int index, int end) {
        checkNotNull(seq);
        if (index < end) {
            char c1 = seq.charAt(index++);
            if (c1 < Character.MIN_HIGH_SURROGATE || c1 > Character.MAX_LOW_SURROGATE) {
                // Fast path (first test is probably all we need to do)
                return c1;
            } else if (c1 <= Character.MAX_HIGH_SURROGATE) {
                // If the high surrogate was the last character, return its inverse
                if (index == end) {
                    return -c1;
                }
                // Otherwise look for the low surrogate following it
                char c2 = seq.charAt(index);
                if (Character.isLowSurrogate(c2)) {
                    return Character.toCodePoint(c1, c2);
                }
                throw new IllegalArgumentException("Expected low surrogate but got char '" + c2 + "' with value "
                        + (int) c2 + " at index " + index + " in '" + seq + "'");
            } else {
                throw new IllegalArgumentException("Unexpected low surrogate character '" + c1 + "' with value "
                        + (int) c1 + " at index " + (index - 1) + " in '" + seq + "'");
            }
        }
        throw new IndexOutOfBoundsException("Index exceeds specified range");
    }

    /**
     * Helper method to grow the character buffer as needed, this only happens once in a while so
     * it's ok if it's in a method call. If the index passed in is 0 then no copying will be done.
     */
    private static char[] growBuffer(char[] dest, int index, int size) {
        if (size < 0) { // overflow - should be OutOfMemoryError but GWT/j2cl don't support it
            throw new AssertionError("Cannot increase internal buffer any further");
        }
        char[] copy = new char[size];
        if (index > 0) {
            System.arraycopy(dest, 0, copy, 0, index);
        }
        return copy;
    }
}
